This invention relates generally to the field of wireless digital communication systems and, more particularly, to wireless digital communication systems that support adaptive modulation and coding schemes.
Wireless communication systems, such as cellular, use a wireless link comprised of a modulated radio frequency (RF) signal to transmit data between sender and receiver. Since RF bandwidth is a scarce resource, various signal processing techniques have been developed for increasing efficiency of the usage of the available RF bandwidth. An example of such signal processing techniques is the IS-95 promulgated by the telecommunication industry association (TIA). The IS-95 standard, used primarily within cellular telecommunications systems, incorporates code division multiple access (CDMA) to carry out multiple communications simultaneously over the same bandwidth. In accordance with the IS-95 standard, data is transmitted over a RF link at a maximum data rate of 9.6 or 14.4 kbps for voice codec, or 64 kbps for data communication, depending on which rate set from a set of data rates is selected. Such data rates as specified by IS-95 may be suited for wireless cellular telephone systems if the typical communication involves the transmission of digitized voice or lower rate digital data such a facsimile.
The International Telecommunication Union (ITU) of the Internet Society, the recognized authority for worldwide data network standards, has recently published its International Mobile Telecommunications-2000 (IMT-2000) standard. The standard proposes so-called third generation (3G) and beyond (i.e., 3.5G, 4G etc.) data networks that include extensive mobile access by wireless, mobile nodes including cellular phones, personal digital assistants (PDAs), handheld computers, and the like. (See http://www.itu.int). The IMT-2000 standard adopts wideband direct sequence code division multiple access (W-CDMA) as a wireless access method for the proposed third generation and beyond networks and requires a maximum data rate of 144 kbps (vehicular), 384 kbps (pedestrian) or 2 Mbps (quasi-stational), depending on the environment in which wireless communication is carried out. Thus, in communication networks according to the IMT-2000 standard, communication services that require high data transmission rates, such as the multimedia communication service, are indeed feasible over RF links.
The recent phenomenal growth of Information Technology and the Internet creates a need for a high performance wireless Internet technology and has in fact promoted development of various data transmission technologies for wireless data services. One such technology is the adaptive data rate scheme in which a data rate is adaptively changed according to the receiver's RF link condition. One of the key requirements for wireless Internet is to maximize the data throughput in a given cell or sector. The adaptive data rate scheme optimizes data throughput on average by serving multiple data receivers simultaneously at maximum data rates that the receivers can accept given their RF link conditions.
The adaptive data rate scheme is a unique technology in many aspects. Recognizing the characteristics peculiar to data services, such as traffic asymmetry and high tolerance to latency, the adaptive data rate scheme decouples data service from voice service. Two-way conversational speech requires strict adherence to symmetry on the downlink (forward link) and uplink (reverse link) traffic and is very delay sensitive. For instance, latencies above 100 ms are intolerable and make speeches unintelligible. It is also true that a relatively modest data rate is sufficient for high quality voice service. On the other hand, data services are characterized by heavy downlink traffic and light uplink traffic and have high tolerance to latency. For high-speed data downlinked at 1 Mbps, for example, 100 ms represents just 100 kb or 12.5 kbytes, and even latencies of a couple of seconds are hardly noticeable. The decoupling of voice and data services reduces design complexities of Physical Layer because it is relieved from difficult system load-balancing tasks, such as one for determining whether voice or data calls have higher priority.
To serve multiple receivers simultaneously at different data rates, the adaptive data rate scheme is usually implemented with time division multiple access (TDMA) scheme. TDMA scheme subdivides the available frequency band into one or several RF channels called “frames.” The frames are further divided into a number of physical channels called “time slots.” The adaptive data rate scheme takes advantage of the characteristics of the TDMA channel that data rate control is possible on each slot. Implementation of the adaptive data rate scheme requires measurement of a RF channel condition and determination of a maximum data rate that the RF channel can accept. For this and other useful purposes, at least one pilot burst is inserted into each time slot. Upon reception of the first pilot burst in each time slot, a receiver estimates the downlink channel condition and computes the maximum data rate that the estimated channel condition can support while maintaining a low error rate. The receiver then reports the calculated data rate to the sender. In order to transmit data to the receiver at the reported data rate, the sender selects a modulation scheme and a coding rate that can achieve data transmission at the reported data rate.
When there are multiple receivers requesting data, the sender needs to have a scheduling functionality (a scheduler) that determines the order in which the receivers are served. Various scheduling algorithms have been proposed and used, yet no algorithms have yet been standardized. Basically, these conventional algorithms try to achieve the same goal, i.e., maximizing the average data throughput. To achieve the goal, these algorithms are designed to serve receivers with better channel conditions more favorably. Thus, under these conventional algorithms, receivers with good channel condition are served first, and receivers with poor channel condition are served later. Also, while serving a receiver, if the receiver's channel condition deteriorates, some of these conventional algorithms stop serving the receiver and start serving another receiver with good channel condition to increase the average data throughput. FIG. 1 shows a simplified graphical representation showing implementation of the adaptive data rate scheme. In FIG. 1, an access point (AP) 1 has three sets of data ready to be transmitted to three access terminals (AT) 2, 3 and 4, respectively. The ATs 2-4 have already measured their RF channel conditions based on the received pilot bursts and sent the AP 1 data rates that they can accept. Suppose that the AT 2 has the best channel condition among them, the AT 3 has the next best condition and the AT 4 is the last. Accordingly, the AT 2 is requesting the highest data rate among them, the AT 3 is requesting a lower data rate and the AT 4 is requesting the lowest data rate. According to the above conventional scheduling algorithms, the AT 2 is served first, the AT 3 is next, and the AT 4 is last as shown in FIG. 1.
Other conventional scheduling algorithms are designed to serve ATs favorably whose channel conditions have recently improved. These algorithms assume that a drop in channel conditions is temporary, and stop serving ATs whose channel conditions just dropped until the channel conditions recover. More specifically, these algorithms send data to an AT that has the highest DRC/R, where DRC is the data rate requested by the AT in a given slot, and R is the average rate received by the AT.
It will however be apparent to those skilled in the art that the above conventional scheduling algorithms are nothing but unfair to ATs with poor channel conditions. It may work satisfactorily if a relatively few number of ATs are requesting data. However, in a situation where a large number of ATs frequently request data, these algorithms will be busy serving those with good channel conditions, and those with poor channel conditions will be left unserved until their channel conditions improve. In other words, in a situation where a large number of ATs frequently request data, the algorithms provide discriminating service under which ATs are served only when their channel conditions are good.